Cleanup backend dockerfile

[eternaltyro]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Describe this PR

• Upgrade pdm and pydebug versions
• Reduce RUN layers for apt steps
• Remove gunicorn workers parameter to allow for WEB_CONCURRENCY env

Review Guide: Notes for the reviewer

Gunicorn worker count is recommended to be 2-4 x $(NUM_CORES) . Default is 1. For production deployments we can rely on WEB_CONCURRENCY envvar to set the number of cores.

Checklist before requesting a review

• 📖 Read the Tasking Manager Contributing Guide: https://github.com/hotosm/tasking-manager/blob/develop/docs/developers/contributing.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.
• 🔠 Does this PR introduce or change any environment variables? If so, make sure to specify this change in the description.

[sonarcloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

Catch issues before they fail your Quality Gate with our IDE extension SonarLint